Dual enhancement in HTC and CHF for external tubular pool boiling – A mechanistic perspective and future directions

Abstract

External tubular pool boiling plays an important role in a range of commercial, industrial, and residential applications including refrigeration and air conditioning, distillation, cryogenics, desalination, and steam generation in power plants. Enhancing the pool boiling performance while increasing the operating heat fluxes results in improved cycle efficiency and reduced equipment size. This approach requires dual enhancement in heat transfer coefficient (HTC) and critical heat flux (CHF) simultaneously. An increase in HTC improves the cycle efficiency in refrigeration evaporators, while increasing CHF permits employment of higher operating heat fluxes. As a means of improving the boiling performance over tubular surfaces, a number of enhancement techniques have been reported in literature. A detailed discussion of each enhancement technique and its underlying mechanisms is presented in this paper. The augmentations have been categorized based on the heat transfer enhancement method. Significant enhancements of up to 4–6-fold over plain tubes in the HTC have been reported with High-Flux, Thermoexcel, and Turbo tubes with somewhat limited enhancement in the CHF of 1–1.9-fold. In refrigeration and air-conditioning applications, the augmented tubes have been used in low heat flux ranges and the focus has been primarily on enhancing the HTC. Recently, new mechanistic approaches have been developed to significantly enhance CHF up to 2–3.8-fold and HTC up to 10–40-fold for pool boiling over flat surfaces. Specific experiments conducted on tubular surfaces have shown these mechanistic approaches to be effective for tubular surfaces as well. Based on these studies, a mechanistic approach for providing significant dual enhancement in both HTC and CHF for external tubular boiling is outlined. Such efforts are expected to enable operating the refrigerant evaporators efficiently (high HTC resulting in low wall superheat) at considerably higher heat fluxes (due to enhanced CHF) to dramatically reduce the size of the evaporators with resultant cost savings and environmental benefits as well from the reduced refrigerant inventory.